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Questions and answers are both stored in the same database table. See here.

ID #20 is the ID of an answer, which is inside a question with ID #8.

The database design goes against the RESTful URL design of /questions/x (since not all posts are questions), so if x is an answer, the devs decided to redirect there instead of bombing it with a 404. You'll notice that when you edit a post, the url is /posts/x/edit regardless of whether it's a question or answer.